Oligohydramnios is defined as too little amniotic fluid. Normal amniotic fluid volume changes with gestational age and ways of accurately estimating it have changed over the years. It has been described as:
- Amniotic fluid volume of less than 500 mL at 32-36 weeks of gestation.
- Maximum vertical pocket (MVP) of less than 2 cm.
- Amniotic fluid index (AFI) of less than 5 cm, or less than the 5th percentile.
Amniotic fluid volume (AFV) increases throughout most of pregnancy, with a volume of about 30 mL at 10 weeks of gestation and a peak of about 1L at 34-36 weeks of gestation. AFV decreases towards term, with a mean AFV of 800 mL at 40 weeks.
Amniotic fluid constantly circulates, with an estimated exchange rate as high as 3,600 mL/hour. Excretion of urine by the fetus is the major source of amniotic fluid production in the second half of the pregnancy. Fluid secreted by the fetal respiratory tract also contributes to AVF.
Fetal swallowing is the major pathway of amniotic fluid clearance in the last half of gestation. Fetal skin is highly permeable in the first half of pregnancy, but becomes keratinised at 22-25 weeks of gestation, significantly reducing transfer.
The mother's fluid balance (and also therefore the fetus') have a major effect on the AFV. Increased maternal fluid intake has been shown to increase the AFV in women with oligohydramnios.
Oligohydramnios is secondary to either an excess loss of fluid, or a decrease in fetal urine production or excretion. Oligohydramnios is usually associated with one of the following conditions:
- Rupture of amniotic membranes (ROM).
- Congenital absence of functional renal tissue or obstructive uropathy.
- Conditions that prevent the formation of urine or the entry of urine into the amniotic sac.
- Fetal urinary tract malformations, including renal agenesis, cystic dysplasia, and ureteral atresia.
- Decreased renal perfusion leading to reduced urine production.
- As a sequela of hypoxaemia-induced redistribution of fetal cardiac output.
- In growth-restricted fetuses, chronic hypoxia results in shunting of fetal blood away from the kidneys to more vital organs.
- Anuria and oliguria lead to oligohydramnios.
- Post-term gestation:
- The cause of decreased amniotic fluid volume (AFV) in post-term pregnancies is unknown.
- The decreased efficiency of placental function has been proposed as a cause, but this has never been confirmed histologically.
- Decreased fetal renal blood flow and decreased fetal urine production have been demonstrated beyond 42 weeks in pregnancies involving oligohydramnios.
Oligohydramnios is a complication in approximately 4.5% of all pregnancies, and severe oligohydramnios is a complication in 0.7% of pregnancies. Oligohydramnios is more common in pregnancies beyond term, because the amniotic fluid volume (AFV) normally decreases at term. It complicates as many as 12% of pregnancies that last beyond 41 weeks.
- Fetal causes include:
- Chromosomal factors.
- Congenital factors.
- Intrauterine growth restriction (IUGR).
- Post-term pregnancy.
- Premature ROM (PROM).
- Fetal demise.
- Placental causes include:
- Twin-to-twin transfusion syndrome (monochorionic twins).
- Maternal causes include:
- Maternal dehydration.
- Uteroplacental insufficiency.
- Diabetes (either pre-existing or gestational diabetes).
- Chronic hypoxia.
- Drug-induced causes include indometacin and angiotensin-converting enzyme (ACE) inhibitors.
- Idiopathic causes are also possible.
Test for systemic lupus erythematosus (causes immune-mediated infarcts in the placenta and placental insufficiency). Other maternal risk factors (including hypertension and diabetes) should also be assessed.
The diagnosis is confirmed by ultrasound. It may be discovered incidentally during routine scanning, or noted during antepartum surveillance for other conditions.
- Suspicion of oligohydramnios may be prompted by discrepancies in sequential fundal height measurements, or by fetal parts that are easily palpated through the maternal abdomen.
- During ultrasound, normal-appearing fetal kidneys and fluid-filled bladder may be observed to rule out renal agenesis, cystic dysplasia, and ureteral obstruction.
- Fetal growth should be checked to exclude intrauterine growth restriction (IUGR) leading to oliguria. Doppler ultrasound could be used to assess placental insufficiency, if suspected.
Measurement of amniotic fluid volume
- The two most commonly used objective methods of determining amniotic fluid volume (AFV) include measurement of the maximum vertical pocket depth and the summation of the depths of the largest vertical pocket in each quadrant, or the amniotic fluid index (AFI). (The pregnant abdomen is divided into four quadrants by using the umbilicus as a reference point to divide the uterus into upper and lower halves, and by using the linea nigra to divide the uterus into left and right halves.) The four measurements are summed to obtain the AFI in centimetres.
- Pockets should be free of fetal limbs and the umbilical cord, although some allow for a single loop of cord to be within the fluid pocket. AFV may be artificially increased if the transducer is not maintained perpendicular to the floor. Excessive pressure on the maternal abdomen with the transducer may lead to an artificially reduced measurement.
- A meta-analysis of randomised controlled trials has concluded that the maximum vertical pocket (MVP) measurement during fetal surveillance seems a better choice. The use of the amniotic fluid index increases the rate of diagnosis of oligohydramnios and the rate of induction of labour without improvement in peripartum outcomes.
Sterile speculum examination
- Sterile speculum examination should be performed to check for ROM. Amniotic fluid may pool in the vagina and a ferning pattern may be observed when fluid from the posterior vault is dried and examined under a microscope.
- Cervical mucus may cause false-positive results (as can semen and blood).
- Nitrazine paper/sticks turn blue. (The amniotic fluid is more basic - pH 6.5-7.0 - than normal vaginal discharge - pH 4.5.)
Management of oligohydramnios is based on gestational age. Transfer to a tertiary referral centre may be appropriate if oligohydramnios is severe.
- Before term:
- Expectant management is often the most appropriate course of action, depending on maternal and fetal condition.
- Ongoing antepartum surveillance (including assessment of fetal growth and follow-up monitoring of amniotic fluid volume (AFV)) is necessary.
- Continuous fetal heart rate monitoring during labour has been advocated for all pregnancies complicated by oligohydramnios.
- At term:
- Delivery is often the most appropriate management.
- With reassuring fetal testing, delivery may be safely delayed on the basis of the parity, the gestational age, the inducibility of the mother's cervix and the severity of the oligohydramnios.
- After term:
The treatment of maternal dehydration with oral or intravenous rehydration has been shown to increase the AFV by 30%.
Increasing the amount of fluid within the amniotic cavity can be accomplished during delivery with the use of amnioinfusion. In this process, warm or room-temperature sodium chloride solution is transcervically infused through an intrauterine catheter.
The Royal College of Obstetricians and Gynaecologists does not recommend amnioinfusion for women with preterm rupture of membranes.
Vesico-amniotic shunts may be used to divert fetal urine to the amniotic fluid cavity in patients in whom a fetal obstructive uropathy is determined to be the cause of oligohydramnios. Although it is effective in reversing oligohydramnios, its ability to achieve sustainable good renal function in infancy is variable. Pulmonary function cannot be guaranteed with restoration of the amniotic fluid volume.
- The earlier in pregnancy that oligohydramnios occurs, the poorer the prognosis. Fetal mortality rates as high as 80-90% have been reported with oligohydramnios diagnosed in the second trimester. Most of this mortality is a result of major congenital malformations and pulmonary hypoplasia secondary to PROM before 22 weeks of gestation. Midtrimester PROM often leads to pulmonary hypoplasia, fetal compression syndrome and amniotic band syndrome. The inspiration of amniotic fluid at regular intervals is probably needed for terminal alveolar development.
- The assessment of amniotic fluid volume (AFV) is important in pregnancies complicated by abnormal fetal growth or intrauterine growth restriction (IUGR). Oligohydramnios is a frequent finding in pregnancies involving IUGR and is most likely secondary to decreased fetal blood volume, renal blood flow and subsequently fetal urine output. Pregnancies complicated by severe oligohydramnios have been shown to be at increased risk for fetal morbidity.
- Pulmonary hypoplasia.
- Fetal compression syndrome.
- Amniotic band syndrome.
- The risk of fetal infection is increased (by the presence of prolonged rupture of the membranes).
Further reading & references
- Amnioinfusion for oligohydramnios during pregnancy, NICE Interventional procedure guidance (2006)
- Baxter JK et al, Oligohydramnios Imaging, Medscape, Apr 2011
- Carter BS et al, Polyhydramnios and Oligohydramnios, Medscape, Feb 2008
- Luton D, Alran S, Fourchotte V, et al; Paris heat wave and oligohydramnios. Am J Obstet Gynecol. 2004 Dec;191(6):2103-5.
- Nabhan AF, Abdelmoula YA; Amniotic fluid index versus single deepest vertical pocket: a meta-analysis of Int J Gynaecol Obstet. 2009 Mar;104(3):184-8. Epub 2008 Nov 30.
- Nabhan AF, Abdelmoula YA; Amniotic fluid index versus single deepest vertical pocket as a screening test for preventing adverse pregnancy outcome. Cochrane Database Syst Rev. 2008 Jul 16;(3):CD006593.
- Verrotti C, Bedocchi L, Piantelli G, et al; Amniotic fluid index versus largest vertical pocket in the prediction of Acta Biomed. 2004;75 Suppl 1:67-70.
- Mozurkewich E, Chilimigras J, Koepke E, et al; Indications for induction of labour: a best-evidence review. BJOG. 2009 Apr;116(5):626-36. Epub 2009 Feb 4.
- Biard JM, Johnson MP, Carr MC, et al; Long-term outcomes in children treated by prenatal vesicoamniotic shunting for lower urinary tract obstruction.; Obstet Gynecol. 2005 Sep;106(3):503-8.
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Dr Hayley Willacy
Dr Hayley Willacy
Dr Helen Huins